Field of the Invention
The field of this invention relates to dispersions of particles in a liquid medium. More particularly, the field of this invention relates to dispersions of natural or synthetic graphite dispersions-in-oil which are suitable as a constituent of a lubricating oil composition.
U.S. Pat. No. 2,176,879 (1939) of F. E. Bartell discloses a composition comprising a dispersion of colloidal or finely divided graphite in a non-flocculated form in an organic liquid. A strongly absorbed stabilizing agent is disclosed to prevent flocculation.
U.S. Pat. No. 4,203,728 (1980) of Richard B. Norton discloses a method for preparing a slurry comprising oil and coal. Suspension stability of the coal particles in the oil is achieved by surface oxidation of the coal particle prior to preparation of the slurry. Surface oxidation of the coal could be achieved by techniques disclosed in Supplemental Volume of the text, "Chemistry of Coal Utilization" edited by H. H. Lowry, John Wiley and Son, N. Y. (1963). One of the techniques involved a fluidized bed of air and coal. Another, involved contacting coal particles with aqueous solutions of alkaline permanganate, hydrogen peroxide, ozone solutions and the like. Air oxidation in the range of 120.degree.-350.degree. C. was disclosed as an effective way to oxidize the surface of such coal particles.
Efforts to prepare stable graphite-in-oil dispersions by fluidized bed oxidation in air were not successful. By stable is meant, that the particles do not flocculate and/or settle out upon standing. To facilitate the process of settling, we used an International Model BE Centrifuge. A 100 ml. sample of the graphite dispersion was placed in the centrifuge and centrifugated at 900 RPM for 18 hours. A 5 ml. sample was syphoned out and its graphite content determined. From this, the percent of graphite retained was calculated and used to compare the stability of different dispersions. Stability as determined by this method is essential to the provision of an economical yield of stable graphite dispersions when the product of the wet grinding step is subjected to centrifugation or settling to remove undesirably large (greater than 6 microns) particles. It is also necessary to ensure, for example, that the lubricating properties of the graphite containing motor oil are not lost due to separation of graphite from the motor oil during long periods of storage, and that no graphite particles of their flocs will plug up pores of oil filters and lubrication passages.
In the Journal of Colloid and Interfacial Sciences 54, pgs. 22-27 (1976), there is disclosed that the stability of carbon black in a non-aqueous system of toluene varied with a molecular weight of a polystyrene dispersing agent. The stability was found generally to increase with increasing molecular weight of the polystyrene and that below a certain molecular weight, e.g., 4.2.times.10.sup.3, the stability was poor.
The requirements for a graphite-in-oil dispersion for use in lubrication are more severe than the requirements for a coal-oil dispersion whose main purpose is transport through a pipeline. Since movement in a pipe helps to maintain the coal suspended, the requirements for a coal slurry are likely to be different from those of a graphite in oil dispersion.
Polymeric dispersants having average molecular weight of about 100,000-700,000 include two subclasses.
The first subclass includes the copolymers of long-chain alkyl methacrylates or acrylates with polar monomers such as N-vinyl pyrrolidone, and N-dialkylamino ethyl acrylamide. Examples include Acryloid 940, Acryloid 954, Acryloid 957 and Texaco TL8103. Some examples of compounds within this first subclass are disclosed in U.S. Pat. No. 3,842,009 (1974) and U.S. Pat. No. 3,506,574 (1970) which are both expressly incorporated herein by reference.
The second subclass includes copolymers of two olefins, such as ethylene and propylene, to which a polar monomer such as N-vinyl pyrrolidone or 2-vinyl pyridine optionally has been graft-polymerized. Examples include Acryloid 1155. The Acryloids are products sold by Rohm & Haas.
Some examples of compounds within this second subclass can be found in U.S. Pat. No. 3,242,075 (1966) which is expressly incorporated by reference herein. Specifically disclosed in U.S. Pat. No. 3,242,075 among others is a linear type olefin polymer having a molecular weight above about 1000 and as high as about 200,000, but preferably a molecular weight in the range of about 30,000 to about 80,000. Polyisobutylene is expressly disclosed. Other examples of compounds within this second subclass can be found in U.S. Pat. No. 3,384,580 (1968) which is expressly incorporated by reference herein. Broadly disclosed as a suitable stabilizing agent in a dispersion for use as a lubricating oil are copolymers involving ethylene and/or propylene or terpolymers involving ethylene, propylene and an unconjugated diene monomer.
As a generally well known principle, viscosities of compositions containing non-spherical particles increase without a dispersing agent as the particle size decreases. This can become extremely severe, to the point of gelation. Heating can also accelerate this process of gelation both during or after a grinding process employed to facilitate dispersion in a liquid medium.
Dispersing agents are known to be somewhat effective in overcoming the above problem. However, shear stable dispersing agents effective in overcoming the problems of increasing viscosity with graphite-in-oil dispersions were not known to the instant inventors prior to the discovery of certain non-polymeric dispersion agents discussed in more detail hereinafter.
Accordingly, a method to prepare a shear stable graphite-in-oil dispersion which is an improvement over the methods previously known is needed.